How do I distribute hair particles evenly using guiding curves? (Ponytail with Hair Dynamics)
Combine a Short Hairstyle with a Dynamic Ponytail
Since the main hair doesn't move, but only the ponytail, you can leave the main hair static and make only the ponytail dynamic (plus dynamic bangs, and another layer of fly-off hair if you want more realism.) It's like a hair extension and fairly easy to do.

Turnaround of the ponytail hairstyle
Preparation

create a little cylinder or &quot;ring&quot; (it's more a disc). It will be the emitter for the ponytail. I've added a Torus object (12 major, 8 minor segments), deleted the inner faces, and grid-filled the hole.


Ponytail hair emitter (&quot;ring&quot;)

place this ponytail emitter (&quot;ring&quot;) behind the head where you want to have the ponytail
Important! Apply the scale to the &quot;ring&quot; and the head because a lot of the hair settings are relative to the emitter's size. You will get weird-looking results in the simulation and in Cycles renderings. Eevee is more error-tolerant.

Create and Comb the Main Hair
Note: The main hair is on a haircap/scalp emitter. The head is a separate object and is bald. All objects have real-world size.

add a Hair Particle System to the haircap/scalp. 500 particles, 8 segments are fine. You can increase/reduce the number of segments in Particle Edit mode (menu Particle → Rekey; particles must be selected.)
Comb the hair. Select the lower hair (everything at the level of the ears in front view, box select) and comb it up in the direction to the &quot;ring&quot;. Tighten the strand of hair guides and try to pull them through the ring.
at the front of the head, pull the hair gently back so the children can't accidentally disappear inside the head (bald spots!)
when you're finished, cut all hair particles (guides) behind the ring.
repeat the combing for the side parts, then for the back top, and finally for the bangs (if you want a parting)
try to layer the hair parts
don't forget you can hide combed parts of the hair so it gets not messed up by further edits. You also can select the roots (menu) and select more segments (keys) with Numbpad++ to easily select single hair guides and comb them.

Create the Ponytail

select the &quot;ring&quot;, in Edit Mode, select the inner faces, and assign them to a Vertex Group &quot;Ponytail&quot; (Ctrl+G). This is where the ponytail will grow.
assign material for the hair (&quot;Ponytail&quot; group) and another material for the outside of the ring (inverted selection)
add a Hair Particle System to the ring emitter, 500 particles, 26cm, 8 segments
use the created Vertex Group &quot;Ponytail&quot; in Particle Properties → Vertext Groups → Density so the hair grows &quot;through&quot; the ring and not on the outside of the ring.
enable Particle Properties → [X] Hair Dynamics, in section Hair Dynamics → Structure increase the random value (0.422). This widens the ponytail a little
Note: the values for Vertex Mass and Stiffness have an effect on the stiffness but I left them at the default settings and just did a little weight painting.


Weight paint of the ponytail

in Particle Edit mode, weight paint the keys (black dots) a little, see screenshot. This spreads the ponytail somewhat, makes it fluffy and it doesn't look completely flat from the side.
run the animation. In about 60 frames the ponytail should have fallen down nicely. It should neither sap off nor stick out.


Simulated hair dynamics of the ponytail
Tweak the Hair Particle Settings

Children, select Interpolated, amount: 80. For the ponytail only: Length: 0.959, Threshold: 0.306. It makes the end of the ponytail a little more shaggy.
adjust the values for Clumping, Roughness, and Kink as you like.
use a small number for Roughness → Random, like 0.021
if the children of the main hair try to overgrow the ring emitter then try to increase Clumping → Clump a bit, e.g. 0.479
Kink works well for the ponytail (used Wave in the example) but select a small Amplitude like 0.02 m so it doesn't &quot;explode&quot;
for Hair Shape use values like Strand Shape: -0.9, Diameter Root: 0.03, Tip: 0.01, Diameter Scale: 0.01 (default). These values are important for Cycles if you don't want to have super fat hair particles.

Animation test: